1. Field of the Invention
The present invention relates to a rotary anode for use in a rotary anode type X-ray tube, and more specifically a rotary anode for use in an X-ray tube for which high output is required, such as a tomograph (hereinafter referred to as X-ray CT) used for medical diagnosis, and a method for manufacturing the same.
2. Description of the Related Art
Conventional rotary anodes for use in X-ray tubes are either made of tungsten only or of a laminated structure comprising tungsten and molybdenum. They are manufactured by the powder metallurgical process. When electron beams are applied to the surface of such an anode to produce X-rays, only 1% of the irradiation energy is converted into X-rays while the remaining 99% is converted to heat. Thus, its surface layer is likely to suffer thermal cracks due to thermal fatigue.
With recent rapid progress in the medical techniques, X-ray CT's are required to operate more accurately and reliably and produce high-power X-rays. Thus, the surface temperature of an anode used in such a CT can reach as high as about 3000.degree. C. The temperature of the entire anode will reach about 1000.degree. C. Thus, the anode tends to suffer thermal cracks due to severe thermal fatigue. This causes X-rays to be dispersed, causing a gradual reduction in the amount of X-rays produced.
There are two solutions to this problem. One is to increase the accumulated heat capacity to promote heat absorption and the other is to increase the revolving speed of the anode. But if the weight is increased to increase the heat capacity of a conventional anode, either formed of a single tungsten layer or having a laminated structure comprising tungsten and molybdenum layers, becomes impossible to increase its revolving speed. Thus, it was impossible to stably produce a high power required for X-ray CTs.
As a rotary anode which is free of these problems and which can produce high-power X-rays required for X-ray CTs, anodes have been proposed which comprise a substrate made of graphite, which is a material known to have a low specific gravity and a large heat capacity, and an X-ray generating layer provided on the graphite substrate and made of tungsten or its alloy. Among the methods for manufacturing such an anode, the chemical vapor deposition process (abbreviated to CVD) by which the X-ray generating layer is formed is considered most advantageous because with this method, the bond strength between the graphite substrate and the X-ray generating layer is stable.
Japanese Examined Patent Publication 47-8263 discloses a basic technique for forming an X-ray generating layer of a tungsten alloy by CVD in which a 0.1-mm-thick X-ray generating layer of a tungsten-rhenium alloy containing 1-35% by weight of rhenium is formed on a graphite substrate and in which an intermediate layer of rhenium is formed to attain a high adhesion between the tungsten-rhenium alloy layer and the graphite substrate. In other words, what is obtained is a structure comprising tungsten-rhenium alloy layer/rhenium intermediate layer/graphite substrate.
When a material gas of rhenium is supplied together with a material gas of tungsten, the rhenium serves, for its high reaction rate, as cores when the crystal grows. Thus, the metallographic structure of the tungsten-rhenium alloy layer becomes fine. Such fine structure shows increased strength and increased recrystallization temperature and thus is more resistant to thermal cracks. But since rhenium as the material gas is extremely expensive compared with tungsten, the above-mentioned technique, which provides a thick tungsten-rhenium alloy layer containing a large amount of rhenium, poses a problem that the rotary anode produced with this technique tends to be prohibitively expensive. Such anodes are therefore not used very widely.
In order to solve this problem, unexamined Japanese Patent Publication 63-228553 proposes a relatively low-cost double-layered X-ray generating layer comprising an ordinary columnar structure made only of tungsten and an overlying layer having a fine structure formed by adding rhenium to tungsten. Namely, the X-ray generating layer obtained with this technique has a structure comprising, from its outer side, tungsten-rhenium alloy layer having a fine structure/tungsten layer having a columnar structure/rhenium intermediate layer/graphite substrate.
But this rotary anode has a problem in that there are points where the distribution of rhenium is discontinued in the X-ray generating layer. Moreover, because of the difference in thermal expansion coefficient between tungsten and rhenium (the thermal expansion coefficient of the former is 4.6.times.10.sup.-6 k.sup.-1 whereas that of the latter is 6.7.times.10.sup.6 k.sup.-1), peeling tends to occur at the discontinuous points in the rhenium composition, i.e. at the interface between the tungsten-rhenium alloy layer and the tungsten layer.
On the other hand, U.S. Pat. No. 4,920,012 discloses another method for providing an X-ray generating layer having a fine metallographic structure. With this method, the feed rate gradient of the material gas in the CVD process is set at 105 cm/cm.sec or higher to form an X-ray generating layer having an equi-axed metallographic structure having an average crystal grain diameter of 0.04- 1 .mu.m. Namely, this X ray generating layer has a structure comprising, from its outer surface, tungsten or tungsten-rhenium alloy layer having a fine equi-axed structure/rhenium intermediate layer/graphite substrate.
This technique makes it possible to provide an X-ray generating layer having a fine structure without adding rhenium. But with this method, the structure tends to grow in a branch-like manner. The film thus obtained is low in mechanical strength and the X-ray generating layer is relatively brittle, so that it is more likely to suffer thermal cracks, which may extend deep into the X-ray generating layer.